The present invention relates to a process for forming a metal film on a surface of a synthetic resin substrate and to an aluminum film coated matter formed by coating a surface of a synthetic resin substrate with an aluminum film.
Heretofore, synthetic resin substrates such as acrylic resin moldings, provided with metal films formed on the respective surfaces thereof, are used in miscellaneous decoration goods, optical parts, optical recording media, etc. Particularly, optical discs such as video discs for which the demand has increased remarkably in recent years, are provided by forming metal films such as aluminum films as reflection films on the respective surfaces of transparent synthetic resin substrates having recording pits corresponding to audio signals, video signals, etc.
The metal films on this type substrates are heretofore formed mainly by a technique of vacuum evaporation. In recent years, an attempt to form the films by a technique of plasma sputtering (parallel plate plasma sputtering) has been made.
The technique of vacuum evaporation is, in general, applied to an arrangement in which an evaporation source 8 and synthetic resin substrates 6 such as acrylic resin moldings opposite to the evaporation source 8 are provided in a vacuum vessel 2 as shown in FIG. 16, and it is a technique for respectively forming films of a metal such as aluminum on the respective surfaces of the substrates 6 by depositing particles 10 of a metal such as aluminum on the respective surfaces of the substrates 6 through evaporating the metal particles 10 from the evaporation source 8 by means of electron beam heating or resistance heating. The substrates 6 may be generally mounted on rotary holders 4 as shown in FIG. 16. To increase the number of substrates to be subjected to film forming, a large number of substrates 6 may be arranged along the circumferential wall of a cylindrical vacuum vessel 2 having an evaporation source 11 in its center as shown in FIGS. 17 and 18. The evaporation source 11 has a large number of tungsten heaters 14 stretched between two props 12, and a large number of evaporation materials 16 such as aluminum attached thereto, so that the respective evaporation materials 16 are evaporated through heat generated by energizing the respective tungsten heaters 14. The reference numeral 18 designates a current input terminal. The substrates 6 are mounted on the holders 4 supported through rotary shafts 22 to holder supports 20, respectively, and they are rotated in the direction of the arrow A as shown in FIG. 17 in the vacuum vessel 2 as a whole.
The technique of plasma sputtering is, in general, applied to an arrangement in which a sputtering source (for example, magnetron type sputtering source) 24 having a metal target 28 such as aluminum attached to the upper portion of a magnet 26 and synthetic resin substrates such as acrylic resin moldings opposite to the sputtering source 24 are provided in a vacuum vessel 2 as shown in FIG. 19, and it is a technique for respectively forming films of a metal such as aluminum on the respective surfaces of the substrates 6 by depositing metal particles on the respective surfaces of the substrates 6 through sputtering the metal target 28 through plasma 30 generated between the sputtering source 24 and the respective substrates 6 by a magnetic field formed in the neighborhood of the surface of the metal target 28, an electric field applied between the respective substrates 6 (in the strict sense, holders 4 therefor) and the metal target 28, and the like, in the vacuum vessel 2 in which an argon gas is introduced.
In the case where films are formed by the conventional technique of vacuum evaporation, however, the adhesion of metal films such as aluminum films to synthetic resin substrates such as acrylic resin moldings exhibits practicable strength but varies so widely as to be deficient in stability. Further, reflectivity is proportional to film thickness when the film thickness is not more than about 600 .ANG.. There arises a problem in that sufficient reflectivity cannot be obtained stably unless each of the metal films such as aluminum films has a film thickness of about 600 .ANG. at the least. This is because the kinetic .energy of evaporated metal particles such as aluminum particles is considered to be so low as about 0.1 eV that the metal films deposited on the substrates are poor in fineness, crystalline arrangement, etc.
In the present state, metal reflection films of aluminum are most widely put into practical use in the field of optical discs such as video discs. In particular, in the case of optical discs, the characteristic (for example, S/N ratio) thereof is determined by the reflectivity of aluminum films formed on pits as well as the form of the pits with respect to signal reproduction. In general, good images cannot be reproduced unless the reflectivity is not less than about 70%. Further, it is necessary in view of the producing process that the formed aluminum films sufficiently adhere to synthetic resin substrates.
An attempt to reduce the film thickness of aluminum films to attain both material saving and reduction of film-forming time to thereby attain improvement in producing efficiency is effective for reduction in cost of optical discs. The conventional technique of vacuum evaporation, however, has a problem in that the aluminum films cannot be thinned so as to be not more than about 600 .ANG. in view of the reflectivity.
Further, the technique of vacuum evaporation requires a large number of work steps because of problems on the lifetime of the resistors (for example, tungsten heaters 14 as shown in FIGS. 17 and 18) for resistance heating in the evaporation source, and on the supply of a metal such as aluminum being an evaporation material. Accordingly, a substantial problem against reduction in cost, that is, limitation in producing efficiency and stability of production, is inherent in the technique of vacuum evaporation. Such a problem is also inherent in a recently developed technique (so-called in-line single disk production system) in which evaporation is applied to substrates one by one, as well as the aforementioned technique (so-called batch type stand alone system) in which evaporation is applied to a large number of substrates at once as shown in FIGS. 16 and 17.
On the other hand, in the technique of plasma sputtering, a continuous operation can be made until the initially arranged metal target such as aluminum is worn out. This is effective for improvement in producing efficiency. Further, sufficient reflectivity can be obtained stably if the film thickness of metal films such as aluminum films is not less than 500 .ANG.. The technique of plasma sputtering, however, has a problem in that the adhesion of metal films, especially, aluminum films, to synthetic resin substrates such as acrylic resin moldings is insufficient.
The lowering of the adhesion of aluminum films is considered to be caused by resin surface deterioration, involving in resin injury due to temperature, produced by exposing surfaces of synthetic resin substrates to high-density plasma at the film-forming time and by some interaction between the aluminum films and impurity gas released from resin surfaces by heat due to inflow of acceleration electrons accelerated to high energy in an electric field, into the substrates as the anode side.
Further, in the case where the aluminum film formed by the aforementioned technique of vacuum evaporation or plasma sputtering is used as a reflection film of an optical information recording medium, the generally obtained performance thereof is heretofore limited to a range of about 72% to about 80% as its reflectivity and a range of about 38 dB to about 41 dB as S/N ratio of the regenerative output. In the case where high picture quality is required, that is, in the case where, for example, the aluminum film is used for reproduction of high definition television images, there arises a problem in that the reproduced images are insufficient in sharpness if the reflectivity and S/N ratio is limited as described above.
In the case where the technique of vacuum evaporation is used, an aluminum film having higher reflectivity (80 to 85%) and higher S/N ratio (41 to 45 dB) can be provided by reducing the amount of oxidized aluminum in the aluminum film. The aluminum film thus provided, however, has a tendency that there occurs easily a durability problem in that both rising of noise and lowering of recording strength are brought by corrosion and the like caused by moisture absorption oxidation and chemical reaction when the aluminum film is left for a long time. Accordingly, there arises a problem in that improvement in both the performance (reflectivity and S/N ratio) of the film and durability thereof cannot be attained simultaneously.